The purpose of this study was to compare arterial blood gas measurements made during the baseline, initial sedation, maintenance, and recovery periods in patients sedated with midazolam and saline (MS), midazolam and fentanyl (MF), diazepam and saline (DS), and diazepam and fentanyl (DF). During induction both the MF and the DF groups had significantly lower average PO2, pH, and O2 saturations and significantly higher PCO2 values than either the MS or the DS group. While the differences were not as great, the same was also true during the maintenance phase of the procedure. During recovery, patients receiving MF had lower average PO2, pH, and O2 saturation and higher PCO2 values than the MS or the DF group. Patients receiving MF had significantly lower average PO2, pH, and O2 saturation levels and significantly higher average PCO2 levels during all three postsurgical periods than at baseline. Patients receiving DF had significantly lower average PO2, pH, and O2 saturation levels and significantly higher Pco2 levels during both induction and maintenance than at baseline.